Translation regulation plays a vital role in the lives of most organisms. It provides an important checkpoint in the pathways for cell growth and differentiation, and a link to the pathology of several diseases. A prominent example of translational regulation in early fly embryogenesis is the repression of maternal hunchback mRNA by Pumilio, Nanos, and Brain tumor. The Nanos protein is only synthesized at the posterior of the embryo due to the translational repression of its own maternal mRNA by Smaug. Pumilio and Nanos are also required for the translation repression of maternal Cyclin B mRNA. Our long-term objective is to uncover the structure and mechanism of assembly of these translational regulators. Specific aims are: 1. Test our basic hypothesis that two Pumilio molecules bind to a single hunchback regulatory element. This will accomplished through a combination of structure and genetics. 2. Identify specific residues in Brain tumor that interact with Nanos. 3. Cocrystallize Pumilio and Nanos in a ternary complex with hunchback mRNA. 4. Determine the structure of Pumilio Puf domain bound to Cyc B mRNA. We will first define the minimal element in the Cyclin B mRNA that binds Pumilio with high affinity and confers regulation in vivo. 5. Elucidate the structure of a SAM domain/RNA complex by NMR methods. 6. Guided by structure, isolate a Smaug dominant negative for the identification of potential corepressors. Together, these structural and genetic experiments will provide a molecular basis for the translation regulatory events that organize the body pattern along the anterior-posterior axis.